[3] Diplomacy in the ancient world had no concept of maintaining permanent embassies on foreign soil -- the role of the proxenos was as close as the Greeks came to this -- so foreign affairs were conducted by ad hoc envoys often deputized on a temporary basis and given very limited scope of powers, typically to negotiate the resolution of a very specific problem.
A presbys was chosen for their respectability and experience in politics and foreign affairs, but they were not given great latitude of powers.
Typically the body that granted them their mission laid down extremely specific and restrictive instructions about how negotiations should be handled.
This is remarked in many ancient sources as being a somewhat ineffective strategy, and could sometimes communicate a lack of unity on a given topic or message, as each presbys in a group would often emphasize the perspective or special interests of their own particular faction.
[2] But they were clearly distinct from the role of keryx, which was a messenger tasked only to deliver information, and clearly had some power to negotiate and agree to terms on behalf of their home countries.